Climatic Research Unit

2007 Posters

The role of eddies in determining the Southern Ocean response to the Southern Annular ModeJ. Screen, N. Gillett, D. Stevens, G. Marshall and H. Roscoe

The Southern Ocean shows a clear response to the Southern Annular Mode (SAM). A positive shift in the SAM is associated with cold sea surface temperature (SST) anomalies centred at 60°S and warm SST anomalies centred at 40°S, and increases in the transport of the Antarctic Circumpolar Current (ACC). It has also been shown that Southern Ocean eddy activity increases when the SAM is in it's positive phase. Eddies may act to reduce the SST response to the SAM by increasing southward heat transport, and reduce the ACC zonal transport response to the SAM by transferring momentum downwards in the water column. Consequently the Southern Ocean response to the SAM may be difficult to capture exactly with climate models that do not resolve eddies explicitly.

Results are presented from the OCCAM ocean model which is run at 3 spatial resolutions - 1°, 1/4° and 1/12° - from coarse to eddy-permitting. Comparing coarse with fine resolutions indicates the effect of explicitly-resolved eddies on the Southern Ocean response to changes in the SAM. The observed SST response to the SAM is well represented by all resolutions of the model. The regressions between monthly Drake Passage transport anomalies and the SAM are not significantly different in the three model resolutions. These results are encouraging for climate modellers as they suggest that low (1°) resolution models are able to capture the main features of the Southern Ocean response to the SAM on monthly timescales. However, preliminary results suggest that the response of the Southern Ocean eddy field to the SAM is lagged by 2-4 years. Consequently, over inter-annual timescales eddies may play a more important role.

There has been strong stratospheric ozone depletion in the Southern Hemisphere since the 1980s. Ozone losses in the Southern Hemisphere occur from 250-20 hPa and are seasonally dependent, with the maximum losses occurring during October. Sonde measurements taken from different Antarctic stations have shown that the ozone depletion occurs later at lower altitudes.

Over the same period as the ozone depletion there has also been an observed downward trend in the geopotential height and temperature over Antarctica in the spring and summer months. The stratospheric trends peak in November, whereas the tropospheric trends are largest in December and January. Surface temperatures are most sensitive to ozone loss near the tropopause, therefore we hypothesise that the observed tropospheric response is forced mainly by ozone depletion in the lower stratosphere (below 160 hPa).

In this study the climate response to ozone depletion is simulated using the higher vertical resolution, 64 level, Hadley Centre atmosphere model coupled to a slab ocean. We have considered changes in the ozone concentration limited to the region near the tropopause to determine the impact of summer ozone radiative forcing in the lower stratosphere. Preliminary results suggest that changes in the lower stratospheric ozone concentration do not have a large impact on the tropospheric climate. This suggests that the tropospheric response is forced mainly by the depletion in the midstratosphere.